“I’ll know it when I see it” is such a challenging phrase. On the one hand, it doesn’t offer any guidance toward the desired direction. On the other hand, that’s sometimes all anyone is able to provide.
I think there are two problems behind this statement
The goal isn’t clear. If there is a clear goal, then the direction is easier to uncover.
It lets you off the hook. If you’re clear about what you’re looking for, and then it doesn’t work out, you could get blamed. By giving the vague “I’ll know it when I see it”, you can always just say “we never found the right solution”.
Anything you create, whether it’s a page on a website, a printed flyer, or a social media post, should have a goal behind it. Some have little goals, some have big goals, but every bit of marketing should be working toward your main objectives. Once you sort that out, “I’ll know it when I see it” starts to fade away.